MdePkg/BaseMemoryLibOptDxe ARM AARCH64: fix thinko in SetMem##

MdePkg/BaseMemoryLibOptDxe ARM AARCH64: fix thinko in SetMem##

The new InternalMemSetMem##() implementations for ARM and AARCH64 in
BaseMemoryLibOptDxe fail to take into account that the 'length' argument
is not in bytes, but in number of items to be copied. So multiply by the
item size before proceeding.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>